The present invention relates to a method for controlling the exposure of stationary mounted video cameras, e.g., for traffic monitoring.
From the British Patent No. 2186 461, a method for controlling the exposure for a stationary video camera is described. At issue, in this context, are cameras which are used in automatic monitoring systems. These camera systems monitor an object on a 24-hour basis. This means that the exposure circumstances for the area being observed can change generally over the course of the day, even drastically. Such camera systems must have an automatic exposure control built in. In the British patent document, a sensor is proposed which detects the light conditions through the camera lens. Using the measured values, the illumination parameters of the camera are set and corrected. The sensor receives its exposure data from an area that is not the visual range of the video camera. This has the disadvantage that under very difficult light conditions predominating in the area to the observed, false exposures can occur. Thus it is possible, for example, that a wet and shiny street requires very different exposure conditions than can be recognized by the sensor from its measurement of an image segment, e.g., of an area of sky. Conventional automatic gain controls permit a continuous adjustment of the exposure parameters through a constant evaluation of the image content.
In contrast, a method according to the present invention has an advantage that the exposure parameters for the video camera are actually obtained from the image segment that the camera observes and actually on the light-sensitive plane. In addition, it is used for applications in which the strong differences of day and night are advantageously compensated for by a flash device. Thus it is possible to obtain a good exposure even in very difficult light conditions, if it is only individual images that are evaluated for adjusting the exposure parameters.
It is particularly advantageous that the method selects measuring zones from the image, and, using gray-scale histograms and cross correlations, determines a correction factor. It is particularly advantageous that the correction factor used for the exposure parameters is calculated as an average value of all the selected measuring zones. Furthermore, it is advantageous that the individual correction factor is monitored for its plausibility. In this way, it is avoided that, e.g., a vehicle moving through the image falsifies the exposure and thus the correction factor. Moreover, it is advantageous that, for the monitoring of an image overload, measuring zones are likewise selected from the image. For the presence of an overload to be confirmed, a plurality of criteria must be satisfied.